Ocean invertebrates

The amazing French film Oceans opens in Melbourne on 26 May. This documentary about the wealth of life in seas was filmed over four years by a global team. MV’s Julian Finn and Mark Norman worked with the film crew as scientific consultants for several of the animals filmed. Two of these animals - Nomura's Jellyfish (Nemopilema nomurai) and a blanket octopus (Tremoctopus gracilis) are often found together in the near-surface waters of the open ocean.

Blanket octopuses are so-named because of the membranous webs that the females possess on two of their arms. This is a defence mechanism: a two-metre-long female blanket octopus can use her webs to mislead potential predators about her size and shape. If this doesn’t intimidate them, she can also shed off pieces of her web – ‘like sheets of toilet paper,’ according to Julian – which in turn stretch out into long, tangling filaments.

Detail of the female Tremoctopus web, showing the bands where bits of it can break off as a defence mechanism.Image: Julian FinnSource: Museum Victoria

Another extraordinary thing about blanket octopuses is the size difference (or dimorphism) between males and females. We discussed size dimorphism on the blog recently but here’s the most extreme example we know of. In Tremoctopus, the male can be up to forty thousand times smaller than the female by weight!

Female Tremoctopus.Image: Julian FinnSource: Museum Victoria

Now from the miniscule to the massive. Nomura’s Jellyfish is one of the largest cnidarians in the word. When these creatures invade Japan’s coastal waters, thousands of jellyfish can clog fishing nets, making the nets so heavy that fishing boats have overturned trying to recover them. Oceans includes footage of Julian diving with one so you can see for yourself just how huge they are.

A still from the film Oceans showing Julian Finn swimming with a giant Nomura's Jellyfish. Source: courtesy of Galatee Films

Julian believes that Tremoctopus are able to survive in hostile environment of the open ocean through association with jellyfish, probably feeding on the small fish that live amongst the tentacles and within the bell of giant Nomura’s Jellyfish. Male and small female Tremoctopus harvest the stinging tentacles of another variety of jellyfish – the Portuguese Man-of-War (Physalia spp.) – to use for their own defence and/or prey capture, suggesting a long association between two quite different types of animals.

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